linux-kernel - Re: [PATCH] Re: bad pmd ffff810000207238(9090909090909090).

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 28 May 2008 22:43:56 +0200
From:	Willy Tarreau <w@....eu>
To:	Jan Engelhardt <jengelh@...ozas.de>
Cc:	Hugh Dickins <hugh@...itas.com>, Fede <fedux@...men.org.ar>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] Re: bad pmd ffff810000207238(9090909090909090).

On Wed, May 28, 2008 at 10:14:31PM +0200, Jan Engelhardt wrote:
> 
> On Wednesday 2008-05-28 21:56, Willy Tarreau wrote:
> >On Wed, May 28, 2008 at 07:36:07PM +0100, Hugh Dickins wrote:
> >> On Tue, 27 May 2008, Fede wrote:
> >> > 
> >> > Today I tried to start a firewalling script and failed due to an unrelated
> >> > issue, but when I checked the log I saw this:
> >> > 
> >> > May 27 20:38:15 kaoz ip_tables: (C) 2000-2006 Netfilter Core Team
> >> > May 27 20:38:28 kaoz Netfilter messages via NETLINK v0.30.
> >> > May 27 20:38:28 kaoz nf_conntrack version 0.5.0 (16384 buckets, 65536 max)
> >> > May 27 20:38:28 kaoz ctnetlink v0.93: registering with nfnetlink.
> >> > May 27 20:38:28 kaoz ClusterIP Version 0.8 loaded successfully
> >> > May 27 20:38:28 kaoz mm/memory.c:127: bad pmd
> >> > ffff810000207238(9090909090909090).
> >> > 
> >> > I also found another post with a very similar issue. The other post had almost
> >> > the same message (*mm*/*memory*.*c*:*127*: *bad* *pmd*
> >> > ffff810000207808(9090909090909090).)
> >> > 
> >> > Does anyone know what is it?
> >> 
> >> Thanks a lot for re-reporting this: it was fun to work it out.
> >> It's not a rootkit, it's harmless, but we ought to fix the noise.
> >> Simple patch below, but let me explain more verbosely first.
> >> 
> >> What was really interesting in your report was that the address
> >> is so close to that in OGAWA-San's report.  I had a look at that
> >> page on my x86_64 boxes, and they have lots of 0x90s there too.
> >> It's just some page alignment filler that x86_64 kernel startup
> >> has missed cleaning up - patch below fixes that.  There's no
> >> security aspect to it: the entries were already not-present,
> >> they just generate this noise by triggering the pmd_bad test.
> >
> >Is there a particular reason we use 0x90 as an alignment filler ?
> 
> Alignment within functions. You could use a JMP to jump over
> the alignment, but that would be costly. So in order to
> "run through the wall", you need an opcode that does not
> do anything, something like 0x90.
> 0xAF would map to scasd on x86, and I'd hardly call that a
> no-op.

OK, I did not understand from Hugh's explanation that it was
all about alignment within functions. Of course, 0x90 is fine
there (though there are multi-byte NOPs available).

Cheers,
Willy

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/